The ultimate goal of three-dimensional display technology is to show a dynamic three-dimensional image. Despite the fact that the human visual system is uniquely constructed to function in a three-dimensional (3-D) world, the majority of current commercially available display technologies are based on flat, two-dimensional (2-D) methods of displaying visual information. In an attempt to display 3-D information on these displays, many sophisticated and powerful techniques, such as perspective, shadowing, and texturing have been developed to trick the eye into seeing a 3-D image. These techniques, however, are ultimately limited in effectiveness because they do not provide depth cues in a manner that is natural to visual processing. Therefore, displays capable of producing a true 3-D image are required to overcome the limitations of 2-D displays.
Volumetric displays form voxels—pixels in a 3-D grid—to represent objects in space. Because each voxel emits or scatters light at a specific point in space, volumetric displays represent objects having a more natural depth perception. Conventional 3-D volumetric displays utilize an upconversion process to produce an image within a volumetric display. One of the fundamental requirements for an upconversion volumetric 3-D display system is to have the entire display volume filled with voxels that can be selectively excited at any desired location. Methods of creating the required voxels include, solid-state (rare-earth particles doped into a glass host), gas medium (atomic vapor enclosed in a vaccum container), and crystal cube static screen (tiny dots engraved within a glass cube using a process known as Laser Subsurface Engraving), among others. To represent an object within the volume, two independently controlled laser beams activate a voxel only when they intersect in a process known as two-photon upconversion. Briefly, this process uses the energy of two infrared (IR) lasers to transform a material from a first level into a first excited level and then to a second excited level, from which it can make a visible fluorescence transition back to the first level. For this process to be useful as a display medium, each voxel must exhibit laser absorption from two different wavelengths, so that it is turned on only at the intersection of two independently scanned laser sources. Either laser by itself will not cause visible fluorescence. However, where both lasers are incident on the same voxel, two-step excitation results in fluorescence at the intersecting point. Scanning the intersection point fast enough, a flicker free 3-D image can be drawn in the volume. The eye cannot see changes faster than about 1.5 Hz. Therefore, if the image to be displayed is repeatedly drawn faster than this rate, the image will appear to be steady, even though visible light may be originating from any one point in the volume only a small fraction of the time.
Upconversion systems, however, are limited because these systems must write a new pattern at every discrete depth, requiring timing, processing, and control computations that limit the refresh rate, especially for large images, and require extensive electronic support systems. In addition, the brightness, color palette and contrast are primarily controlled by one or more high brightness, single-color laser sources or by a nonlinear optical process that produces a 3-D image from within the image space. Both methods restrict the range of colors in the image and increase the cost of the display, and the nonlinear process has difficulties in producing images bright enough to see in typical room illumination.
To overcome the rendering limitations of the above referenced volumetric displays, some systems have employed high quality 2-D images projected onto an opaque screen which was mechanically driven by a rotary or oscillating mechanism. The screen may have been rotated about an axis, or oscillated, for example, but the surface was always facing a fixed direction. As a result, the display would sweep a display space distributing frames of a 2-D image depending on the angle of the screen and the viewing angle. Different shapes of screens have been employed, including flat, helical, and thin elongated members such as a rod, beam, or stick. While these systems allowed full-color, high-contrast images to be produced, the refresh rate, size of the display, and viewing angle were limited by the mechanical nature of the system.
Some attempts to produce 3-D visual displays have also employed light emitting diodes (LEDs), for instance, on rotating blades. Like other systems, rotating LED systems have some desirable qualities, but, again, they have limitations that have kept them from being widely adopted to produce 3-D or holographic effects.
As a result of the above referenced limitations, there remains a need for systems and methods for easily producing bright, full-color, high-contrast 3-D images that are viewable continuously and equally over a wide range of viewing angles without the need for a mechanically driven opaque screen.